Monday, February 22, 2016

Sega Golden Axe Bootleg Fix Log

I pulled a Golden Axe Bootleg PCB out of storage. I hadn't fired it up for 20 years or so.

Unfortunately, it didn't work. The screen was too dark, apart from when the game was performing its windowing effect, where it suddenly displayed at perfect brightness. Sometimes the screen would completely lose sync and go black.

Initially, I didn't hold out much hope. Whilst there are few customs on this boardset compared with the official Sega PCB, there are hundreds of ICs and no schematics.

Top PCB

Bottom PCB

I started by inspecting the board. There was a Fujitsu IC on the top board that was running hotter than the surface of the sun. As Fujitsus have a bad rep, I swapped it out. The new IC ran cooler, but the windowing problem persisted.

I moved around the board shorting pins with my logic probe in an attempt to figure out what area of the board was responsible for the rendering issue.

I struck gold and found an IC with a damaged leg. It was my lucky day. This was data input pin 6 of an 74LS169 chip. This is a 4-bit binary counter. It would make sense for this counter logic to be used in the windowing effect when resizing the display area. To test this theory I quickly connected the damage leg back up and the problem vanished. This was going to be an easy fix!

I socketed and replaced the IC. I powered up the board. It was now completely dead! I tried a different 74LS169 chip. Still completely dead! I wished at this point I hadn't snipped the legs of the original LS169 and had simply repaired it in-situ.

Suspecting my handiwork fitting the IC socket, I resoldered it for a second time. The board remained completely dead. I was baffled.

I then swapped in a different LS169. The board booted!

I can only presume that a combination of a faulty LS169 I had purchased and some bad soldering on my part caused the board to play dead. Unfortunately I do not have a way to test LS169s ICs out of circuit. The faulty replacement was a 74LS169BN as opposed to a 74LS169AN, but I'm assured they are compatible.

The bootleg has a number of issues, which are probably the result of it being a bootleg as opposed to a fault on the PCB itself:

- The screen momentarily fills with garbage for a frame or so on screen clear
- The sound isn't as clear as an original PCB
- There is no service mode available to test the RAMs and ROMs